Copolyester-carbonate resins are well known thermoplastic materials which, due to their many excellent properties, are finding increasing use as thermoplastic engineering materials in many commercial and industrial applications. These polymers have outstanding mechanical, thermal and optical properties such as high tensile strength, optical clarity, thermal and dimensional stability and impact strength. The copolyester-carbonates are typically prepared by reacting (i) a carbonate precursor, (ii) a difunctional carboxylic acid, and (iii) a dihydric phenol to provide a linear polymer consisting of units of the dihydric phenol linked to one another through carbonate and ester linkages.
These copolyester-carbonates differ from most thermoplastic polymers in their melt rheology behavior. Most thermoplastic polymers exhibit non-Newtonian flow characteristics over essentially all melt processing conditions. Newtonian flow is defined as the type of flow occurring in a liquid system where the rate of shear is directly proportional to the shearing force. However, in contrast to most thermoplastic polymers, copolyester-carbonates prepared from dihydric phenols exhibit substantially Newtonian flow at normal processing temperatures and shear rates.
Two other characteristics of molten thermoplastic polymers are considered to be significant for molding operations: melt elasticity and melt strength. Melt elasticity is the recovery of the elastic energy stored within the melt from distortion or orientation of the molecules by shearing stresses. Melt strength may be simply described as the tenacity of a molten strand and indicates the ability of the melt to support a stress. Both of these characteristics are important in extrusion blow molding, particularly in fabrication by extrusion blow molding. Non-Newtonian flow characteristics tend to impart melt elasticity and melt strength to polymers thus allowing their use in blow molding fabrication. In the usual blow molding operation, a tube of molten thermoplastic is extruded vertically downward into a mold, followed by the introduction of a gas, such as air, into the tube thus forcing the molten plastic to conform to the shape of the mold. The length of the tube and the quantity of material forming the tube are limiting factors in determining the size and wall thickness of the objects that can be molded by this process. The fluidity of the melt obtained from conventional copolyester-carbonates such as those derived from bisphenol-A, or the lack of melt strength as well as the paucity of extruate swelling, serve to limit blow molding applications to relatively small, thin walled parts. Temperature must generally be carefully controlled to prevent the extruded tube from falling away before its attains the desired length and the mold is closed around it for blowing. Consequently, the Newtonian behavior of linear conventional copolyester-carbonate resin melts has severely restricted their use in the production of large hollow bodies by conventional extrusion blow molding operations as well as the production of various other shapes by profile extrusion methods.
There thus exists a need for copolyester-carbonate resins which lend themselves to blow molding operations. Branched copolyester-carbonates lend themselves to blow molding operations. A process for preparing branched copolyester-carbonates from (a) a carbonate precursor, (b) a dihydric phenol, and (c) a difunctional carboxylic acid is disclosed in U.S. Pat. No. 4,286,083. However, the branched copolyester-carbonates obtainable by this process contain a relatively low ester content. It is an object of the instant invention to provide branched copolyester-carbonates containing a relatively high ester content.